48 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS another appropriate odour. Thus, methyl ionones--which are very powerful, persistent odours rapidly leading to fatigue will obliterate some odours but not others. This type of masking is really "overpowering." Some odours will neutralise each other so that the characteristic odour of neither can be smelt. This is not necessarily due to chemical interaction or to physical association between them, but is a form of masking known as "neutralisation." Iodoform with balsam of Peru and musk with bitter almonds are often-quoted pairs (this is not well substantiated), and cyclo- hexanol and cineole are another pair which have a camphoraceous smell when blended in the correct proportions. (j) .Fixation. Addition of relatively small amounts of some, usually not very volatile, substances to volatile odorants can reduce their rate of evapora- tion, making them last longer and thereby blend into the less volatile odorants dissolved with them. PART II RELEVANT INFORMATION ABOUT OTHER SENSES Many authors have theorised on the subject of smell and, with the exception of Piesse, have thought it to be related to the visual sense, drawing conclusions which amount to postulating the existence of a small number of primary smells and regarding smell as a "chemical sense"--a chemical sense being one in which a stimulant gives rise to a primary chemical reaction which is converted by a suitable mechanism to a message sent along a nerve to the brain. A physical sense is one which causes an inductive, positional or mechanical change, such as vibration or undulation, of some resonating mechanism which is then converted to a message. Sight is concerned with the response of from one to four chemicals to radiation, the chemicals absorbing quanta of energy, undergoing chemical change, thereby giving rise to a nerve message: it is thus a chemical sense. Hearing is a sense in which vibrations of molecules are transmitted to vibra- tions of hairs, giving rise to the appropriate nerve responses: thus it is a physical sense. Smell could be either a chemical or a physical sense. Molecules of odorants could, on the one hand, react with some sensitive chemical in the olfactory organ, giving rise to a chemical change and associated message to the brain, the chemical change being subsequently reversed by a third reactant to recover the original responding chemical (a favourite mechanism of the nervous system). On the other hand, the odorant molecules could be adsorbed on, or dissolved in, some part of the olfactory organ to change its physical position or state, resulting in a nerve message, the receptor recover-

SMELL--THE PHYSICAL SENSE 49 ing its original state by mechanical removal of the odorant molecules by the air-stream flowing over it. In the former case it would be a chemical sense analogous to sight, in the latter a physical sense analogous to hearing. If a chemical sense, then a limited small number of receptor chemicals would be expected, giving rise to primary odours from which all other odours could be compounded in this case it would probably be impossible to differentiate between the ingredients of a composite odour for the same reason that the eye cannot differentiate between the primary colours making up a composite colour. If smell is a physical sense, we should expect to find no such primary odours and to be able to differentiate between the ingredients of a composite smell in the same way that the ear can analyse a composite sound. It will be advantageous to consider in greater detail the sensory nerve systems, the visual and auditory senses, and the structure of the olfactory area before attempting to explain any observed phenomena. We will also consider the properties of gases relevant to an adsorption theory of smell. The following are brief descriptions of these. (a) The Sensory Nerve System. A sensory nerve system consists of (i) the receptor organ, (ii) the nerve which transmits, via a limited number of junctions, the response to the stimulus given to the receptor, and (iii) the receiving area of the brain specialising in the interpretation of the message. Receptor organs are of several kinds. Those responsible for touch, light pressure, heat and cold are all constructed on the same basis as Pacinian corpuscles, which are those responding to pressure by change of shape they are large receptors made of concentric shells arranged like layers of onion with a soft core through which the nerve passes, to end at the far side. There are other kinds of touch receptors, such as Meissner corpuscles, which are ellipsoids, the nerve ending inside the soft centre in a complex •nesh of fine fibres. Yet another end-organ is associated with hair follicles, in which the nerve ending is grouped basket-like round the base of the hair so that the nerve cell is stimulated by mechanical displacement of the hair. This latter receptor device is used in the ear to convert sound into nerve signals, and in the vestibular apparatus which controls the balance of the body. The nerves are bundles of fibres rarely greater than 0.01 mm. in diameter, each fibre being an extension of the nerve cell which sends out two fibrils-- one to the receptor, the other towards the brain. Nerve fibres are of two types: medulated and non-medulated. A medulated fibre consists of a senfi- fluid central core surrounded by a sheath of concentric layers of protein (myelin sheath) which in turn is covered by a homogeneous membrane called "the neurilemma." Non-medulated fibres resemble medulated, except that the myelin sheath is absent. When the nerve is excited, an electric current passes along it, the active region of the nerve becoming negatively charged with respect to the rest